This proposal focuses on elucidating the function of Raf Kinase Inhibitor Protein, RKIP, a novel kinase inhibitor that was identified as a potent tumor metastasis suppressor in vitro, and whose transcription regulation - by Snail and/or EZH2 (Enhancer of Zeste Homolog 2) - we hypothesize is key to understanding signaling pathways involved in prostate cancer progression and metastasis. Mechanistically, this protein functions as a negative regulator of both the Raf and NF-_B signaling pathways. Consistent with its inhibitory effects on these two pathways, a significant inverse correlation was observed between the expression of RKIP and the stage of cancer development in prostate tumors. Particularly, high levels of RKIP were noted in healthy prostate tissue, whereas these levels progressively decreased to almost undetectable levels in prostate tissues of increasing aggressiveness and metastatic capability. Importantly, restoration of RKIP expression in highly metastatic prostate cancer cell lines sensitized them to apoptosis and inhibited metastasis in a xenograft mouse model, suggesting this protein as a promising candidate for cancer therapy. In order to utilize this potential, the present application proposes to define the transcription factors that regulate RKIP expression (Specific Aim #1) and establish its physiological relevance in vivo (Specific Aim #2). Particularly, Specific Aim #1 will examine the possible roles of the transcription factors Snail and EZH2 in regulating RKIP expression in prostate cancer. This will be done by both loss-of-function and gain-of-function approaches, as well as through a combination of genetic and biochemical methods. Specific Aim #2 will study a possible in vivo role of Snail and EZH2 as regulators of RKIP expression in prostate cancer, using DNA and tissue microarray (TMA) studies to correlate between the expression of RKIP and the two abovementioned transcription factors in samples from prostate cancer patients. Carcinoma of the prostate is the most common malignancy among males in the US. This proposal should provide a new handle for understanding its etiology in molecular detail and initiate studies aimed at dissecting the complex regulatory transcriptional network that is responsible for downregulation of RKIP. The information gained from this research offers a promising potential for both prostate cancer therapy, as well as other cancers.